1. Field of the Invention
The present invention relates in general to power generation. In particular although not exclusively the present invention relates to systems and methods for the production of power utilising geothermal energy.
2. Discussion of the Background Art
Given the increased awareness of the need for controlling emissions of CO2 has seen considerable investment in clean/green technology. One of the largest sources of CO2 emissions comes from the generation of power and more from coal powered stations. There are presently are number of power generation technologies which have a significantly lower carbon footprint to that of fossil fuel powered stations.
One such alternative is nuclear power, nuclear power provides about 6% of the world's energy and 13-14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity. While nuclear power is a sustainable energy source that reduces carbon emissions, it is exceedingly controversial. As recent examples in Japan and those of Chernobyl and Three Mile Island have shown the threat of meltdown is an ever present concern.
Another concern with nuclear power plants is the production of nuclear waste. A typical 1000-MWe nuclear reactor produces approximately 20 cubic meters (about 27 tonnes) of spent nuclear fuel each year (but only 3 cubic meters of vitrified volume if reprocessed). Spent nuclear fuel is initially very highly radioactive and so must be handled with great care and forethought.
However, it will decrease with time. After 40 years, the radiation flux is 99.9% lower than it was the moment the spent fuel was removed from operation. Still, this 0.1% is dangerously radioactive. After 10,000 years of radioactive decay, according to United States Environmental Protection Agency standards, the spent nuclear fuel will no longer pose a threat to public health and safety.
When first extracted, spent fuel rods are stored in shielded basins of water (spent fuel pools), usually located on-site. The water provides both cooling for the still-decaying fission products, and shielding from the continuing radioactivity. After a period of time (generally five years for US plants), the now cooler, less radioactive fuel is typically moved to a dry-storage facility or dry cask storage, where the fuel is stored in steel and concrete containers.
In addition to the problems of meltdown and waste there are also security concerns. Nuclear reactors and waste dumps are prime targets for terrorist, cause a meltdown and you can take out a large populated area and spread radioactive materials across a wider radius. The waste itself is also a target as it can be used in the manufacture of dirty bombs etc.
An alternate approach to nuclear power is that of geothermal power generation. Electricity generation from geothermal power requires high temperature resources that can only come from deep underground. The heat must be carried to the surface by fluid circulation. This circulation sometimes exists naturally where the crust is thin: magma conduits bring heat close to the surface, and hot springs bring the heat to the surface. Until recently most geothermal electric plants have been built exclusively where high temperature geothermal resources are available near the surface. The development of binary cycle power plants and improvements in drilling and extraction technology may enable enhanced geothermal systems over a much greater geographical range.
Enhanced Geothermal Systems (EGS) are a new type of geothermal power technologies that do not require natural convective hydrothermal resources. Until recently, geothermal power systems have only exploited resources where naturally occurring heat, water and rock permeability is sufficient to allow energy extraction from production wells. However, the vast majority of geothermal energy within reach of conventional techniques is in dry and non-permeable rock. EGS technologies “enhance” and/or create geothermal resources in this hot dry rock (HDR) through hydraulic stimulation.
When natural cracks and pores will not allow for economic flow rates, the permeability can be enhanced by pumping high pressure cold water down an injection well into the rock. The injection increases the fluid pressure in the naturally fractured rock which mobilizes shear events, enhancing the permeability of the fracture system. This process, termed hydro-shearing [3], used in EGS is substantially different from hydraulic tensile fracturing used in the oil & gas industries.
Water travels through fractures in the rock, capturing the heat of the rock until it is forced out of a second borehole as very hot water, which is converted into electricity using either a steam turbine or a binary power plant system. All of the water, now cooled, is injected back into the ground to heat up again in a closed loop. EGS/HDR technologies, like hydrothermal geothermal, are expected to be baseload resources which produce power 24 hours a day like a fossil plant. Distinct from hydrothermal, HDR/EGS may be feasible anywhere in the world, depending on the economic limits of drill depth.
In either case the thermal efficiency of geothermal electric plants is low, around 10-23% because geothermal fluids are at a low temperature compared with steam from boilers. By the laws of thermodynamics this low temperature limits the efficiency of heat engines in extracting useful energy during the generation of electricity. HDR wells are expected to have a useful life of 20 to 30 years before the outflow temperature drops about 10° C. and the well becomes uneconomic. If left for 50 to 300 years the temperature will recover. This limited life span and the expenses of drilling etc. makes the use of such power stations economically undesirable limiting their application.
Clearly it would be advantageous to provide a system and method for power generation which has a relatively low carbon footprint and which ameliorates some problems associated with the aforementioned prior art.